1. Field of the Invention
The present invention relates to a secondary battery having an ion conductive member, and a manufacturing process thereof. More particularly, it relates to a secondary battery in which the decomposition of an electrolyte solution attributed to the repetition of charge and discharge is suppressed.
2. Related Background Art
Since the quantity of carbon dioxide gas contained in the atmosphere is currently increasing, the warming of the earth is predicted to increase due to the greenhouse effect. It has, therefore, become difficult to build new thermal power stations which emit carbon dioxide gas in large quantities. In this regard, for the purpose of effectively utilizing electric power generated by a dynamo of a thermal power station or the like, so-called xe2x80x9cload levelingxe2x80x9d has been proposed, wherein night power is stored in secondary batteries installed in general households and is used in the daytime when large power consumption is required, thereby leveling a station load. Meanwhile, the development of a secondary battery of higher energy density has been expected in the field of electric vehicles, which features the emission of no air-pollutive substances. Further, it is of urgent necessity to develop high-performance secondary batteries which are smaller in size and lighter in weight for use in the power sources of portable apparatuses such as notebook type personal computers, word processors, video cameras and pocket telephones.
As the high-performance secondary battery of smaller size and lighter weight, an example wherein a lithium-graphite interlayer compound is applied to the negative electrode of a secondary battery was reported in JOURNAL OF THE ELECTROCHEMICAL SOCIETY, 117, 222 (1970). Since then, by way of example, a rocking chair type secondary battery, i.e., a so-called xe2x80x9clithium ion batteryxe2x80x9d has been developed wherein a carbonaceous material is employed as a negative-electrode active material, while an interlayer compound doped with lithium ions is employed as a positive-electrode active material, and wherein lithium is introduced and kept between the layers of the carbonaceous material by a charge reaction. Some articles of this type are being put into practical use. With the lithium ion battery, the carbonaceous material as a host material which is intercalated between the layers using lithium as a guest is applied to the negative electrode, thereby suppressing the growth of the dendrite of lithium during charge and achieving a longer lifetime in charge and discharge cycles.
However, an organic solvent is used as the solvent of an electrolyte solution in the secondary battery utilizing galvanic reactions (charge and discharge reactions) based on lithium ions, such as the lithium-ion secondary battery. Therefore, when the battery is overcharged, the solvent is decomposed thereby producing carbonic acid gas, hydrocarbons, etc., and it is not restored to its original state by a recombination reaction. It is accordingly believed that the electrolyte solution will deteriorate thereby increasing the internal impedance of the secondary battery. Further, the overcharge of the secondary battery causes internal short circuiting thereof, along with the generation of heat and promotion of a reaction which rapidly decomposes the solvent, which can lead to the breakage of the battery.
In order to prevent the secondary battery from degrading as stated above, the battery is sometimes furnished with an overcharge prevention circuit, a PTC (Positive Temperature Coefficient) element whose resistance increases with the rise of temperature or the like. This contrivance, however, increases cost.
Besides, in order to solve the drawbacks of the decomposition and deterioration of the electrolyte solution in the secondary battery which utilizes the charge and discharge reactions based on lithium ions, U. S. Pat. No. 5,609,974 has proposed a secondary battery which adopts a solid polymer electrolyte, obtained in such a way that monomers of three typesxe2x80x94a diacrylate type, a monoacrylate type, and an acrylate type including a carbonate groupxe2x80x94are copolymerized in the presence of both an organic solvent and a supporting electrolyte and in which coke and a lithium cobalt oxide are respectively applied to a negative electrode and a positive electrode. The solid polymer electrolyte, however, exhibits an ionic conductivity which is below xc2xc as compared with that of a liquid electrolyte in which a supporting electrolyte is dissolved in a solvent. Consequently, a current density in the secondary battery is limited, and an energy density is also low.
On the other hand, an electrolyte solidifying technique which prevents liquid leakage while avoiding degradation in performance to the extent possible has been required also of a high-performance alkaline storage battery (secondary battery) which uses a hydrogen-occlusion alloy or the like for a negative electrode.
The present invention has been made in view of the problems stated above, and it has for its object to provide an electrolyte for use in a secondary battery, which is immune against deterioration and decomposition in the charge and discharge reactions of the secondary battery and a novel secondary battery which employs the electrolyte.
The first mode of the present invention consists of a secondary battery wherein an ion conductive member is arranged between a positive electrode and a negative electrode which are disposed in opposition to each other, characterized in that said ion conductive member has its ion channels oriented so as to have a higher ionic conductivity in a direction of joining a plane of said positive electrode and that of said negative electrode. Herein, the secondary battery shall cover a contrivance wherein said ion conductive member has a layered structure or a columnar structure.
The second mode of the present invention consists of a process for producing a secondary battery wherein an ion conductive member is arranged between a positive electrode and a negative electrode which are disposed in opposition to each other, characterized by orienting ion channels of said ion conductive member so that said ion conductive member may have a higher ionic conductivity in a direction of joining a plane of said positive electrode and that of said negative electrode. Herein, the process for producing the secondary battery shall cover a contrivance wherein said ion conductive member is endowed with a layered structure or a columnar structure.
The ion conductive member which constitutes the secondary battery according to the present invention can be endowed with the layered or columnar structure. In that case, ion conducting paths (ion channels) along which the migrating distances of ions become substantially the shortest are formed in a direction parallel or perpendicular to the layered or columnar structure. Therefore, the ionic conductivity of the ion conductive member becomes the highest in the paths, and the ion conductive member exhibits an anisotropic conductivity. In this regard, according to the secondary battery of the present invention, the direction in which the ionic conductivity of the ion conductive member is higher is brought into agreement with the direction which is perpendicular to the planes of the negative and positive electrodes opposing one another. Thus, the secondary battery of the present invention can have its internal resistance lowered, and it is permitted to be charged and discharged at a higher efficiency and a higher current than any secondary battery which does not adopt the ion conductive member of such a structure.
In addition, the ion conductive member of the layered or columnar structure should preferably be a polymer gel electrolyte which is formed in such a way that a polymer serving as the matrix of the specified structure is caused to absorb an electrolyte solution (a solution obtained by dissolving a supporting electrolyte in a solvent).
The process for producing a secondary battery in the second mode of the present invention can be performed by sandwiching the ion conductive member endowed with the layered or columnar structure between the negative and positive electrodes. The ion conductive member is fabricated in such a way that a cross-linked polymer material having the layered or columnar structure is prepared and is thereafter caused to absorb an electrolyte solution, or that a cross-linked polymer material having the layered or columnar structure is prepared in the presence of an electrolyte. The cross-linked polymer material having the layered or columnar structure can be obtained in such a way that the molecules of a cross-linking polymer are arrayed into a regular arrangement by at least one operation selected from the group consisting of irradiation with light, application of a magnetic field, application of an electric field, and heating, whereupon the resulting molecules are cross-linked. Alternatively, the polymer of the specified structure can be obtained in such a way that a compound which has a molecular structure serving as a template is employed in the operation of preparing the cross-linked polymer.